A preliminary investigation of the geothermal resource potential of the San Pedro River valley was undertaken the latter part of 1980. The area of investigation is situated in Pinal, Pima and Cochise counties, Arizona. It extends from the town of Mammoth, Pinal County, south-southeast along the river valley to just north of the Johnny Lyon Hills, an area comprising 2331 km2 (900 mi 2). With the exception of mining and smelting activities in the Mammoth-San Manuel area, the primary business in the valley is agriculture, cattle ranching and forage crops. The majority of the irrigation, livestock, and domestic wells are along the bed of the San Pedro River, an ephemeral stream that flows northward from its headwaters in Mexico. The wells generally vary in depth from 24 m (80 ft) to 36 m (120 ft) and essentially produce from the subsurface .